In the study, researchers from King's College London analysed 2,066 millennial twins in England and Wales. They were asked about their experiences at different stages of life, their relationships, how lonely they were, and how their mental and physical health was.

The most recent data was taken while the twins were 18. Results showed that 7% of participants said they often felt lonely, while 23-31% said they felt left out or lacked companionship.

Overall, being lonely was associated with double the chance of a mental health problem, and a 38% increased risk of being unemployed. Also, lonely individuals were more likely to engage in risky behaviours like smoking and not exercising.

"If somebody discloses to their friends or family, or a GP, that they feel lonely a lot of the time, that could be a warning sign that they are struggling in other areas of life," Timothy Matthews, co-author of the study from King's College London, told the Guardian.

If you're using them to keep in contact, it's less likely to be a problem. But if you spend most of your time scrolling aimlessly and feeling jealous about what everyone else is doing, you're more likely to get FOMO and feel lonely.

In an article for Forbes, Caroline Beaton discussed another possible reason that millennials are so lonely, in addition to spending more time on the internet.

Loneliness, she said, can be contagious. A study from 2009 showed how people are 52% more likely to be lonely if they are connected to someone who is lonely. Also, people who aren't lonely tend to become lonelier around someone who is.

"Lonely people are less able to pick up on positive social stimuli, like others' attention and commitment signals, so they withdraw prematurely - in many cases before they're actually socially isolated," she wrote. "Their inexplicable withdrawal may, in turn, make their close connections feel lonely too."

The lonely can also act a little hostile, possibly as a defence mechanism. This further severs their social ties and perpetuates their loneliness.

The researchers said this new study's findings could help "underscore the importance of early intervention to prevent lonely young adults from being trapped in loneliness as they age."

The participants were only 18, and it's very possible their loneliness levels will change when they grow up. But for those who struggle until later on in life, further research could help them figure out why it is.

"Feeling lonely isn't in itself a mental health problem, but the two are strongly connected," Stephen Buckley from the mental health charity Mind told The Guardian.

"Being sociable and connecting with other people is rewarding in its own right and can help significantly improve your mental well-being, especially if you're not feeling so good."